The present invention relates to a computerized method for the planning and delivery of radiation therapy. In particular, it is a computerized method that determines the optimal radiation treatment plan for a patient using specified clinical objectives.
In general, radiation therapy is the use of ionizing radiation for the treatment of disease. The most common use is in the treatment of cancer. The goal of radiation therapy in cancer treatment is to destroy any malignant cells while minimizing the damage to healthy tissue. One example of a device for delivering the radiation to a patient is a linear accelerator (LINAC). A LINAC is a machine that generates a high-energy beam of radiation that can be controlled and directed onto specified locations. LINACs are sometimes equipped with a multi-leaf collimator (MLC). A MLC is a device that shapes each individual beam of radiation. Alternatively, a LINAC may not be equipped with a MLC, but instead use four collimation jaws that can be individually controlled to shape a rectangular radiation field.
FIG. 1 is an exemplary diagram of a background art radiation therapy system 2. The radiation therapy system of FIG. 1 shows the external structure of a LINAC with a rotating gantry 6 mounted on a stationary stand 9. A LINAC waveguide (not shown in FIG. 1, but normally located inside the gantry 6) accelerates the electrons to produce high energy radiation beams. The gantry 6 swivels the LINAC on a horizontal axis of rotation 8 around the patient 13 who is lying on a bed 16. The LINAC is a device capable of controlled delivery of radiation to target 12 in the body of the patient 13 being treated by the radiation therapy system 2. The radiation exits through the end of the treatment head 4 that is mounted on the gantry 6. In some LINACs, the treatment head is equipped with a MLC (not shown in FIG. 1, but normally located inside the treatment head 3) that can shape the radiation fields that exit the treatment head 4 into arbitrary shapes. In some radiation therapy systems 2, the treatment head 4 is equipped with simpler collimator jaws (not shown in FIG. 1, but normally located inside the treatment head 4) instead of a MLC. In most cases, each collimator jaw can move independently to shape a rectangular field. In addition, the radiation therapy system 2 has a control unit 200 in a housing 18 at least one display unit 70 and a keyboard 19.
During treatment, the LINAC emits a beam of radiation 10 is aimed at the target 12 in the body of the patient 13. As a non-limiting example, the beam of radiation can be photons, electrons, or any other type of radiation used in background art radiation therapy systems. The gantry 6 can rotate about a horizontal axis of rotation 8 around the patient 13 and thus allows for a change in the angle of the beam of radiation 10.
FIG. 2 is an exemplary diagram of a background art MLC 401. The MLC 401 is shown in FIG. 2 is a beam shielding device that is provided within the treatment head 4 (FIG. 1) and is in the path of beam of radiation 10. The MLC 401 beam shielding device determines the irradiated field that impinges the target 12 in the body of the patient 13 (FIG. 1). As shown in FIG. 2, the MLC 401 may include a plurality of leaves 41a, 41b . . . 41n, 42a, 42b . . . 42n, though only one pair 41a, 42a is discussed for convenience. In addition, as shown in FIG. 2, a plurality of drive units 43a, 43b . . . 43n, 47a, 47b . . . 47n, is used to position the plurality of leaves 41a, 41b . . . 41n, 42a, 42b . . . 42n, though only one pair 43a, 47a is discussed for convenience. Moreover, additional pairs of leaves (not shown) may be arranged perpendicular to the plurality of leaves 41a, 41b . . . 41n, 42a, 42b . . . 42n. It is to be understood that the discussion is applicable to the plurality of leaves and the plurality of drive units.
The leaves 41a, 42a are moved by a drive unit 43 in order to change the size of the irradiated field. The drive units 43a, 47a may further include, for example, an electric motor (not shown) that is coupled to the leaves 41a and 42a and is controlled by a motor controller (not shown) that is provided by the control unit 200 (FIG. 1). In addition, position sensors (not shown) may also be coupled to the leaves 41a and 42a, respectively. The MLC 401 arrangement includes the plurality of leaves 41a, 41b . . . 41n, 42a, 42b . . . 42n for blocking radiation and can approximate shapes other than a rectangle. The plurality of leaves 41a, 41b . . . 41n, 42a, 42b, . . . 42n are relatively narrow and cast a shadow of about 0.5 to 1.0 cm in width onto the target 12 (FIG. 1). The more leaves that are used, the more complicated the MLC 401 becomes.
Treatment planning for conventional cancer radiation treatment is often performed with the aid of three-dimensional images of the patient acquired by using a computed tomography (CT) scanner. Based on these images, a radiation oncologist can pinpoint the location of a target tumor and any surrounding sensitive structures. Based on this information, a treatment planner (e.g., a dosimetrist) devises the configuration of radiation beams that will deliver the desired radiation dose to the patient. The parameters that need to be determined by the dosimetrist include: (1) beam energies, (2) beam orientations and (3) field shapes.
Conventionally, using a trial-and-error approach, the dosimetrist determines an acceptable configuration of the various parameters that meets the clinical goals specified by the radiation oncologist. In this technique, referred to as “forward-planning,” the dosimetrist and the radiation oncologist devise the configuration of radiation beams, the computer control unit proceeds according to their direction. If the dosimetrist and radiation oncologist are not satisfied with the radiation dose distribution, they will redefine alternative beam energies, beam orientations and field shapes until a satisfactory result is produced. Therefore, in “forward-planning” approach, human being determines the parameters that produce the best treatment plan.
While background art treatment planning using the “forward-planning” technique for conventional cancer radiation treatment has achieved some success, shaping the radiation field alone provides limited freedom in shaping the volume of the high radiation dose to conform to the tumor. As a result, adverse effects can arise in the patient being treated because of irradiation of normal structures in the “forward-planning” technique to treatment planning.
A recent development in radiation therapy is intensity-modulated radiotherapy (IMRT). In IMRT, the intensity of the radiation is modulated within each field delivered. The purpose of IMRT is to sculpt the radiation dose distribution in order to maximize the radiation dose to the cancerous tumor while limiting the radiation dose to normal structures within some pre-specified tolerance. In IMRT, highly conformal dose distributions can be achieved through the delivery of optimized non-uniform radiation beam intensities from each beam angle. Successful delivery of IMRT can allow for an escalation of the radiation dose to the target tumor that can enhance local tumor control. These dosimetric advantages of IMRT can also be used to provide a reduced probability of complications due to adverse effects due to radiation exposure on normal tissue.
Due to the complexity of treatment plans for IMRT, a computer is used to determine radiation intensity maps that produce an optimal radiation dose distribution. In contrast to the “forward planning” techniques of the background art that use a human to determine the parameters, this approach is termed “inverse-planning” because the computer determines the parameters that produce the optimal radiation treatment plan.
For IMRT treatment delivery, the desired radiation intensity map derived by the computer is converted into overlapping field shapes, called sub-fields or field segments. By delivering these field segments along the same beam angle, the patient receives the desired intensity distribution. This series of beam shapes are delivered either dynamically or in a step-and-shoot fashion.
In most background art intensity-modulated delivery techniques, it is necessary that the LINAC be equipped with a MLC. As shown in FIG. 2, the MLC is a complicated device that is generally made of a plurality of tungsten leaves driven by a plurality of drive units equipped with motors. The cost of a MLC is a substantial portion of the cost of the radiation delivery device (i.e., the LINAC).
However, as discussed above, not all LINACs are equipped with a MLC due to the substantial cost involved. In fact, older LINACs currently in use are not equipped with a MLC but only with collimator jaws. In the case of a LINAC without a MLC, radiation field shaping is achieved with just the collimator jaws. The collimator jaws are two pairs of tungsten blocks. In most cases, at least one pair of jaws can move independent of the other to form fields that are asymmetric with respect to the central axis of the beam. This type of radiation blocking device is often referred to as “independent jaws.”
A limitation of these independent collimator jaws is that these radiation blocking devices can only form rectangular fields. Due to this limitation, the delivery of IMRT using collimator jaws alone was thought to be impractical. In a paper entitled “Intensity-modulated radiation therapy using only jaws and a mask,” by Webb, a technique that places a relocatable mask below the collimator jaws was proposed as a practical approach for the delivery of IMRT treatments. However, use of such a relocatable mask introduces different practical issues and adds cost. Thus, to date there is no known treatment planning system for planning IMRT treatments using collimator jaws alone that does not include practical implementation problems and increased costs.
Due to the reasons stated above, background art IMRT treatment planning systems typically rely on the use of MLCs. These background art techniques are typically performed in two steps: (1) optimizing the radiation intensity maps; and (2) converting the radiation intensity maps to deliverable field segments. However, these background art techniques for IMRT treatment planning typically perform the optimizing step without consideration of the delivery constraints of either the MLC or the LINAC.
Non-limiting examples of the delivery constraints of the MLC include: the speed of leaf travel; the ability to close the opposing leaves; and the ability to slide the opposing leaves across each other. Non-limiting examples of the delivery constraints on the LINAC include the dose rate and the minimal amount of radiation that can be delivered with acceptable accuracy. Ignoring these delivery constraints of the LINAC and MLC in the optimizing step of the treatment plan makes the second step of converting from the intensity maps to deliverable field segments more complex. As a result of this increased complexity, a large number of field segments are required to produce the desired radiation intensity distribution.
In addition, background art IMRT treatment planning techniques rely on the division of the radiation field segments into beamlets. The size of these beamlets is typically 1 cm×1 cm. In the background art, the radiation intensities (i.e., weighting) of the beamlets are typically determined by either turning-on or turning-off beamlets. Thus, the boundary of the resultant radiation beam used to treat a target tumor is constrained to some multiple of the size of the beamlets (e.g., 1 cm×1 cm). For example, a spherical tumor with a 2 cm diameter can only be treated with a square radiation field of beamlets with 2 cm sides (i.e., a square field composed of four 1 cm×1 cm beamlets). If such small tumors are very close to a critical healthy tissue structure, the square radiation field of beamlets will cover both the target tumor and adjacent healthy tissue structures. Therefore, as a result of the size of the beamlets, the shape of the square radiation field segment fails to conform to the shape of the target tumor.
Further, the use of beamlets in background art treatment planning systems also fixes the orientation of the collimator to the orientations of the grid lines. Since the target tumor can be viewed from different beam angles in the beam's eye view (BEV), a fixed collimator would not allow the MLC to best conform to the shape of the tumor. This limitation of the MLC further increases the complexity of delivery constraints that should be considered in treatment planning systems.
In summary, the key techniques in the radiation therapy systems of the background art include the two-step approach of: (1) obtaining the intensity maps; and (2) converting the intensity maps into deliverable field segments. In addition, the division of the beam portal defines the radiation beam's BEV for each radiation beam angle into a set of finite-sized beamlets. As discussed above, each of the background art techniques has certain practical limitations. Further, the use of collimator jaws alone further constrains the shape of the radiation field segments to rectangles. These finite-sized rectangular field shapes may require hundreds of rectangular fields to achieve a desired dose distribution. Such a large number of rectangular fields would require hours of time to deliver and would clearly be impractical.
Therefore, there is a need in the art for more practical, cost efficient and time effective methods of inverse-planning treatment for IMRT. In particular, there is a need for a method for using independent collimator jaws alone that overcomes the limitations of the background art.